This invention relates to an anesthesia vaporizer for introducing anesthetic vapor into the breathing gas supplied by an anesthesia machine to a patient undergoing surgery, and more particularly relates to such a vaporizer capable of being electrically controlled.
A typical anesthetic vaporizer 10 of the prior art is illustrated in FIG. 1, connected to a patient's breathing-system 20. The output of a pulmonary ventilator 12 is also connected, via an absorber head 14 and check valves 16 and 18, to the patient's breathing system 20. Such a ventilator 12 and absorber head 14 is described in more detail in my copending application Ser. No. 685,906 filed Dec. 24, 1984. In addition to anesthetic, muscle relaxants or paralyzing drugs are commonly dispensed to most patient's undergoing surgery. A paralyzed patient requires forced breathing assistance by a pulmonary ventilator. Depending upon the needs of the particular patient however, an anesthesia vaporizer may be used with or without a pulmonary ventilator.
In the prior art vaporizer 10, a carrier gas 22, usually made up of oxygen and nitrous oxide is transmitted through a conduit 24 that includes a restrictor portion 26. A central portion of the conduit 24 including restrictor portion 26 is bypassed, i.e. paralleled, by a long pipe shown as a coil of pipe 28 and a vaporizing chamber 30. Chamber 30 may contain a heated reservoir of liquid anesthetic. In the space above the liquid, the atmosphere of carrier gas 22 is saturated with anesthetic, which saturated gas enters pipe 24 downstream of the restrictor 26.
The concentration of anesthetic in the carrier gas exiting the pipe 24 varies greatly as the pressure at the patient's breathing system varies because the breathing system pressure variations are also reflected at the output of the pipe 24 in the vaporizer.
At the time of high pressure in the breathing cycle, the anesthetic leaves the vaporizer at a low rate. At a subsequent moment of low pressure in the breathing system a glut of anesthetic is introduced into the absorber head and will be delivered in a concentrated pulse to the patient's breathing system in the next inspiratory period. This problem stems from the fact that the flow rate of the carrier gas through the vaporizing chamber 30 is affected by the patient's breathing-system pressure. The saturated vapor concentration is also a function of the temperature. Furthermore, the concentration of anesthetic vapor in the carrier gas is a function of the carrier gas flow rate. A bypass valve 31 is adjusted by the anesthesiologist to establish and maintain the desired average concentration of anesthetic vapor in the carrier gas as is measured and displayed by the vapor concentration monitor 32.
It is an object of this invention to provide an anesthesia vaporizer capable of producing a substantially constant anesthetic concentration regardless of breathing pressure and temperature variations.
It is a further object of this invention to provide such an anesthesia vaporizer wherein the anesthetic concentration is controllable by an electrical signal.